1. Field of the Invention:
The present invention relates to a liquid crystal display device for displaying a great deal of character information and various kinds of images. More particularly, it relates to a projection type liquid crystal display device having a plurality of twisted nematic type liquid crystal layers for displaying images by projecting light through each liquid crystal layer onto a screen.
2. Description of the prior art:
A projection type liquid crystal display device in which a plurality of twisted nematic type liquid crystal panels are used has been developed as a liquid crystal display device for displaying a great deal of character information and various kinds of images. FIG. 7 shows the structure of such a conventional projection type liquid crystal display device, which comprises a liquid crystal display element 6 composed of a pair of laminated liquid crystal panels 60 and 70. Light emitted from a light source 31 is made parallel by light converging lens 32 and projected onto the liquid crystal panel 60. The light penetrates through and is modulated into a given display pattern by the liquid crystal panel 60 and penetrates the other liquid crystal panel 70 to be projected onto a screen 35 by the projecting lens 34, so that a given image can be displayed on the screen 35.
In the liquid crystal panel 60 that is located on the side of the light source 31, a twisted nematic type liquid crystal layer 61 is sandwiched between a pair of substrates 62 and 63 as shown in FIG. 8. In the other liquid crystal panel 70 located on the side of the screen 35, a twisted nematic type liquid crystal layer 71 is likewise sandwiched between a pair of substrates 72 and 73. The liquid crystal layer 61 in the liquid crystal panel 60 on the side of the light source 31 is of STN (supertwisted nematic) type or SBE (Supertwisted Birefringence Effect) type, wherein the twist angles of the liquid crystal molecules are about 180 to 270 degrees. The liquid crystal molecules of the liquid crystal layer 71 in the other liquid crystal panel 70 on the side of the screen 35 is of STN type or SBE type, wherein the rotation direction of twist of the liquid crystal molecules is opposite to that of the liquid crystal molecules in the liquid crystal layer 61 of the liquid crystal panel 60 on the side of the light source 31.
The liquid crystal layer 61 of the liquid crystal panel 60 on the side of the light source 31 is operated by a multiplex drive, and one substrate 62 thereof is provided with a number of transparent scanning electrodes 64 which extend in a parallel manner in a given direction, as shown in FIG. 9. Each scanning electrode 64 is connected to a scanning-side drive circuit 66, and all the scanning electrodes 64 are driven by the scanning-side drive circuit 66. A number of signal electrodes 65 are arranged on the other substrate 63 which is disposed across the liquid crystal layer 61 from the substrate 62, so that the signal electrodes 65 can be arranged at right angles to each scanning electrode 64, resulting in a matrix electrode between the signal electrode 65 and the scanning electrodes 64 placed on the other substrate 62. Each signal electrode 65 is divided into two sections in the direction of arrangement of the scanning electrodes 64. A given voltage is selectively applied to each of the divided signal electrodes 65 by means of signal-side drive circuits 67a and 67b.
On the contrary, substrates 72 and 73 on the liquid crystal panel 70 arranged on the side of the screen 35 are not provided with such electrodes as shown in the above.
In such a liquid crystal display device, voltage is selectively applied to each scanning electrode 64 in the liquid crystal panel 60 on the side of the light source 31 in sequence by the scanning-side drive circuit 66. Voltage is also selectively applied to the signal electrodes 65 in accordance with the display patterns by the signal-side drive circuit 67a or 67b. The light is then modulated at the portions (pixels) of the liquid crystal layer 61 corresponding to the points of intersection of both the electrodes to which voltage is applied. The light penetrates through the liquid crystal panel 60 through to the liquid crystal panel 70 on the side of the screen 35. The liquid crystal panel 70 compensates the coloration of the light penetrating through the liquid crystal layer 61 of the liquid crystal panel 60 at the side of the light source 31, said coloration arising by the effect of birefringence of the liquid crystal layer 61.
Such a construction as mentioned above ensures that the scanning electrodes 64 allow scanning of four hundred lines, for instance, at a duty drive of 1/200.
In recent years, demand is for liquid crystal display devices that have pixels formed with a high density so as to increase the capacity of display.
In a conventional liquid crystal display device of the above-mentioned multiplex drive-type, in order to increase the density of pixels, when the number of the scanning electrodes is increased to thereby increase the multiplex amount (i.e., the number of time-sharing), sufficient voltage is not applied to the liquid crystal pixels and the contrast of images is lowered, or the response speed is lowered, so that the quality of the display is remarkably spoiled. Such a lowering of the contrast and a lowering of the response speed can be prevented if non-linear type elements such as diodes, etc., and switching elements such as thin film transistors, etc., are added to each liquid crystal pixel. However, the ratio of opening, which is the ratio of the area of pixels to the display surface of the liquid crystal layer, is lowered, and the economical efficiency is also lowered.
The signal electrodes and the scanning electrodes are usually formed on substrates by patterning a transparent conductive film such as ITO (indium oxide). Accordingly the electrodes on a substrate become crowded if the number of scanning electrodes is increased in order to increase the density of pixels, so that the patterning itself and/or the connection of the drive circuits to the electrodes becomes difficult.
Moreover, in a direct sight type liquid crystal display device in which a plurality of liquid crystal layers having voltage-applying means respectively, are laminated, there is a difference in sight between the display portions of the laminated liquid crystals. That is, this kind of display device lacks a continuity of the display portions, resulting in a lowering of the quality of display.